Xu Linfeng, Fang Hongjun, Li Sen, Zhu Jinbo, Pan Chengling, Pan Yusong, Feng Qi
Institute of Environment-friendly Materials and Occupational Health (Wuhu), Anhui University of Science and Technology, Wuhu, Anhui 241000, China.
School of Earth and Environment, Anhui University of Science and Technology, Huainan, Anhui 232001, China.
Langmuir. 2020 Apr 28;36(16):4477-4495. doi: 10.1021/acs.langmuir.9b03920. Epub 2020 Apr 13.
A series of samples including leaf-like and rod-like rutile TiO nanoparticles with various facets exposed on the surface, parallelepiped-shaped anatase nanoparticles with [111] vertical facet exposed on the surface, irregular anatase nanoparticles, microsized six-point star-like anatase aggregates, and almond-like brookite aggregates had been hydrothermally synthesized from lepidocrocite-type layered titanate nanosheets. A systematical investigation was established to uncover the phase transition and morphological evolution from nanosheets to TiO polymorphs, and a phase diagram was determined by adjusting the synthesis parameters of the pH value and temperature. Two kinds of mechanisms composed of the dissolution-deposition process following Ostwald's ripening mechanism and the topochemical conversion process following Ostwald's step rule had been proposed based on the time-dependent hydrothermal experiments. Briefly, the formation of the single-crystalline rutile phase appeared only at high temperatures with very low pH values, and similarly, the brookite phase strictly formed at high temperatures with a very high pH value. Nevertheless, the anatase phase could moderately appear in a wide range of temperatures and pH values. In addition, the single-crystalline rutile adopted a leaf-like morphology at low temperatures with high pH values and a rod-like morphology at high temperatures with low pH values, while the morphological evolution of anatase particles proceeded from irregular to parallelepiped-shaped and finally to six-point star-like morphology, and the crystal size was reduced from 1000 to 5 nm with decreasing pH values. Meanwhile, with the prolongation of the hydrothermal time, the layered titanate nanosheets first dissolved into the amorphous state and further converted into small anatase nanoparticles and finally to rutile or anatase nanoparticles based on the dissolution-deposition process, or the {010}-faceted layered titanate structure first converted into the [111]-vertical faceted anatase nanosheets by the topochemical transformation reaction and then split into the [111]-vertical faceted anatase nanoparticles. More importantly, the mesoporous [111]-vertical faceted anatase nanoparticles exhibited enhanced photocatalytic performance compared to that of Degussa P25, which was ascribed to its superior electronic band structure and effective charge separation. The systematical investigation in this work would be significant for consummating the preparation of the TiO polymorphs from layered titanate nanosheets and provided some reference values and guide schemes for the preparation of TiO nanoparticles with outstanding photocatalytic performance.
已通过水热法从纤铁矿型层状钛酸盐纳米片合成了一系列样品，包括表面暴露有不同晶面的叶状和棒状金红石TiO₂纳米颗粒、表面暴露有[111]垂直晶面的平行六面体状锐钛矿纳米颗粒、不规则锐钛矿纳米颗粒、微米级六星状锐钛矿聚集体以及杏仁状板钛矿聚集体。开展了一项系统研究以揭示从纳米片到TiO₂多晶型物的相变和形态演变，并通过调节pH值和温度等合成参数确定了相图。基于随时间变化的水热实验，提出了由遵循奥斯特瓦尔德熟化机制的溶解 - 沉积过程和遵循奥斯特瓦尔德阶梯规则的拓扑化学转化过程组成的两种机制。简而言之，单晶金红石相仅在高温和极低pH值下出现，同样，板钛矿相严格在高温和极高pH值下形成。然而，锐钛矿相能在较宽的温度和pH值范围内适度出现。此外，单晶金红石在低温高pH值下呈叶状形态，在高温低pH值下呈棒状形态，而锐钛矿颗粒的形态演变从不规则到平行六面体状，最终变为六星状形态，且随着pH值降低晶体尺寸从1000 nm减小到5 nm。同时，随着水热时间的延长，层状钛酸盐纳米片首先溶解为无定形状态，然后基于溶解 - 沉积过程进一步转化为小的锐钛矿纳米颗粒，最终转化为金红石或锐钛矿纳米颗粒，或者{010}面的层状钛酸盐结构首先通过拓扑化学转化反应转化为[111]垂直面的锐钛矿纳米片，然后分裂成[111]垂直面的锐钛矿纳米颗粒。更重要的是，与德固赛P25相比，介孔[111]垂直面的锐钛矿纳米颗粒表现出增强的光催化性能，这归因于其优异的电子能带结构和有效的电荷分离。这项工作中的系统研究对于完善从层状钛酸盐纳米片制备TiO₂多晶型物具有重要意义，并为制备具有优异光催化性能的TiO₂纳米颗粒提供了一些参考值和指导方案。
